Study of the daily and seasonal atmospheric CH 4 mixing ratio variability in a rural Spanish region using 222Rn tracer
The ClimaDat station at Gredos (GIC3) has been continuously measuring atmospheric (dry air) mixing ratios of carbon dioxide (CO 2) and methane (CH 4), as well as meteorological parameters, since November 2012. In this study we investigate the atmospheric variability of CH 4 mixing ratios between 2013 and 2015 at GIC3 with the help of co-located observations of 222Rn concentrations, modelled 222Rn fluxes and modelled planetary boundary layer heights (PBLHs). Both daily and seasonal changes in atmospheric CH 4 can be better understood with the help of atmospheric concentrations of 222Rn (and the corresponding fluxes). On a daily timescale, the variation in the PBLH is the main driver for 222Rn and CH 4 variability while, on monthly timescales, their atmospheric variability seems to depend on emission changes. To understand (changing) CH 4 emissions, nocturnal fluxes of CH 4 were estimated using two methods: the radon tracer method (RTM) and a method based on the EDGARv4.2 bottom-up emission inventory, both using FLEXPARTv9.0.2 footprints. The mean value of RTM-based methane fluxes (FR_CH 4) is 0.11 mg CH 4 m −2 h −1 with a standard deviation of 0.09 or 0.29 mg CH 4 m −2 h −1 with a standard deviation of 0.23 mg CH 4 m −2 h −1 when using a rescaled 222Rn map (FR_CH 4_rescale). For our observational period, the mean value of methane fluxes based on the bottom-up inventory (FE_CH 4) is 0.33 mg CH 4 m −2 h −1 with a standard deviation of 0.08 mg CH 4 m −2 h −1. Monthly CH 4 fluxes based on RTM (both FR_CH 4 and FR_CH 4_rescale) show a seasonality which is not observed for monthly FE_CH 4 fluxes. During January–May, RTM-based CH 4 fluxes present mean values 25 % lower than during June–December. This seasonal increase in methane fluxes calculated by RTM for the GIC3 area appears to coincide with the arrival of transhumant livestock at GIC3 in the second half of the year.